like this:
Well, I haven't investigated this properly, but completely interior maps should be possible. Sort of.
You'd need an adapted urban (or the like) terrain (the BLANKS, instead of the usual lawn, changed into a roof-helipad), and a completely new map set, built to mimic parts of the same whole building (it will be hard to deal with the randomization, it'll take no small amount of thinking-mapping-testing).
It's a lot of work and they'll still have a weird look in the end. So I frankly think that for a mod that has consistently all-interior missions, it would be better to wait for the eventual option to override the geoscape-defined terrain.